1. Field of the Invention
The present invention relates generally to surface analysis systems and, more particularly, to a high-precision surface finish, displacement, and contour scanner.
2. The Prior Art
Generally, a surface finish, displacement, and contour scanner, frequently referred to as a probe, is designed for use in a surface analysis system which is used to measure, compute, display and record linear profile and surface finish characteristics, for displacement measuring without traverse, and to perform electronic gauging. "Traverse", as used in this specification and in the appended claims, is meant to define a relative stylus motion which is parallel to the surface being measured. Such surface analysis systems are well known in the art and have been employed for a variety of laboratory and quality-control tasks. Their general applications, for example, include the surface measurement of the inside and the outside diameters of manufactured parts, of gear-teeth profiles, of grooves and of flats. Surface analysis systems readily detect such surface irregularities as bell mouth, runout, taper, roughness and waviness. Surface analysis systems also compute, display and record roughness averages (Ra) of measured surfaces. Roughness average (Ra) is an arithmetical average deviation, usually expressed in microinches or micrometers, and measured normal to the surface. Surface analysis systems are especially useful for measuring deposits on thick and thin-film microelectronic components, i.e., for electronic gauging.
An arm 100 and transducer 102 of a typical prior art probe is shown in FIG. 1. The arm 100 employs a stylus 104 and is mounted cantilever style in a casing, not shown. The transducer 102 includes a thin, flat high permeability blade 106 at the end of the arm 100 that is designed to be accommodated within three collinear slots 110 of an E-shaped ferrite core 108 or other suitable material. The blade 106, accommodated as it is within the slots 110, functions as a movable "flux carrier" whose slightest relative displacement within the slots 110 effectively changes the area of the air gap to control the relative magnitudes of flux produced in two magnetic circuits 114, 116 by an AC-energized primary winding 112, in a manner similar to the operation of the transducer disclosed in U.S. Pat. No. 2,631,272, granted Mar. 10, 1953 to Smith. In short, when the blade 106 is centered vertically in the slots 110, the lines of flux between the two poles of the upper secondary winding 114 and the lines of flux between the two poles of the lower secondary winding 116 are equal. The result is that the difference in signal output between the two secondary windings is zero. As the blade 106 moves upwardly or downwardly from the center position, the lines of flux between the poles of the upper winding 114 change from that of the lower winding 116. The result is that the signal output from the upper winding 114 differs from the signal output of the lower winding 116. Whether the difference is positive or negative depends upon the excitation signal of the primary winding 112 and whether the blade 106 is moving upwardly or downwardly. In any case, if an upward movement causes a negative difference, a downward movement causes a outputs of the two windings 114, 116 is used to determine the displacement of the stylus 104.
A shortcoming of this prior art design is the care needed to manufacture and calibrate the device. In order to ensure symmetry in the secondary winding outputs, the parameters of the collinear slots 110 must be carefully controlled. This results in an added burden in the manufacturing process. Also, the slots 110 are made very narrow to produce a larger change in secondary winding output for smaller movements of the blade 106. It is more difficult to manufacture and calibrate the device when the thickness of the blade 106 approaches the width of the slots 110. Finally, the narrowness of the slots 110 makes it more likely that the blade 106 will make contact with the core 108. Not only does this affect the electrical properties of the transducer 102, but it adds mechanical friction to the movement of the stylus 104, further affecting the measurement.